Import: allow selecting specific audio files

Add support for selecting individual audio files during manual and bulk imports and pass that selection through the scan, API, job queue, processor and organizer.

Key changes:
- API: scan now returns audioFiles for each discovered book and emits a new 'grouping' progress phase; execute and manual-import routes accept file lists (audioFiles / selectedFiles) and validate them.
- Scanner: group loose audio files by metadata (title/author/narrator), deduplicate multi-part sets (CD1/CD2) across folders, and return audioFiles + groupingKey; add concurrency limit for ffprobe reads and merge groups post-scan.
- Job queue & processor: OrganizeFiles payload now includes selectedFiles; processors forward selectedFiles to the FileOrganizer and to cleanup logic.
- File organizer & cleanup: filter to only selectedFiles when organizing; cleanup now deletes only the selected files (if provided) instead of removing the whole directory.
- UI: Manual import browser and bulk import wizard updated to show per-file selection, track checkedFiles, toggle all, and send selected files to the API; ConfirmPhase updated to allow checking/unchecking files and prevents starting import with no files selected.
- Filesystem browse: removed expensive per-subfolder stats to keep browsing responsive (now lists subdirectories without nested stat calls).

Overall this change enables finer-grained imports, reduces accidental deletion of unselected files, and improves scan grouping for multi-folder audiobooks.

src/app/api/admin/manual-import/route.ts

@@ -55,9 +55,25 @@ export async function POST(request: NextRequest) {
         const fs = await import('fs/promises');
 
         const body = await request.json();
-        const { folderPath, asin, cleanupSource } = body;
+        const { folderPath, asin, cleanupSource, selectedFiles } = body;
         let { audiobookId } = body;
 
+        // Validate selectedFiles if provided
+        if (selectedFiles !== undefined) {
+          if (!Array.isArray(selectedFiles) || selectedFiles.length === 0) {
+            return NextResponse.json(
+              { error: 'selectedFiles must be a non-empty array of file names' },
+              { status: 400 }
+            );
+          }
+          if (!selectedFiles.every((f: unknown) => typeof f === 'string')) {
+            return NextResponse.json(
+              { error: 'selectedFiles must contain only strings' },
+              { status: 400 }
+            );
+          }
+        }
+
         // Validate required fields
         if ((!audiobookId && !asin) || !folderPath) {
           return NextResponse.json(
@@ -120,13 +136,52 @@ export async function POST(request: NextRequest) {
           );
         }
 
-        // Verify folder contains audio files
-        const audioCheck = await hasAudioFiles(normalizedPath);
-        if (!audioCheck.found) {
-          return NextResponse.json(
-            { error: 'No audio files found in the selected directory' },
-            { status: 400 }
-          );
+        // Verify selected files exist and are audio files, or fall back to folder scan
+        let audioFileCount: number;
+        const validatedFiles: string[] = [];
+
+        if (selectedFiles && selectedFiles.length > 0) {
+          for (const fileName of selectedFiles as string[]) {
+            // Prevent path traversal
+            if (fileName.includes('/') || fileName.includes('\\') || fileName === '..' || fileName === '.') {
+              return NextResponse.json(
+                { error: `Invalid file name: ${fileName}` },
+                { status: 400 }
+              );
+            }
+            const ext = pathModule.extname(fileName).toLowerCase();
+            if (!(AUDIO_EXTENSIONS as readonly string[]).includes(ext)) {
+              return NextResponse.json(
+                { error: `Not an audio file: ${fileName}` },
+                { status: 400 }
+              );
+            }
+            try {
+              const fileStat = await fs.stat(pathModule.join(normalizedPath, fileName));
+              if (!fileStat.isFile()) {
+                return NextResponse.json(
+                  { error: `Not a file: ${fileName}` },
+                  { status: 400 }
+                );
+              }
+              validatedFiles.push(fileName);
+            } catch {
+              return NextResponse.json(
+                { error: `File not found: ${fileName}` },
+                { status: 404 }
+              );
+            }
+          }
+          audioFileCount = validatedFiles.length;
+        } else {
+          const audioCheck = await hasAudioFiles(normalizedPath);
+          if (!audioCheck.found) {
+            return NextResponse.json(
+              { error: 'No audio files found in the selected directory' },
+              { status: 400 }
+            );
+          }
+          audioFileCount = audioCheck.count;
         }
 
         // Resolve audiobook by ASIN if audiobookId not provided
@@ -317,9 +372,16 @@ export async function POST(request: NextRequest) {
 
         // Queue organize_files job
         const jobQueue = getJobQueueService();
-        await jobQueue.addOrganizeJob(requestId, audiobookId, normalizedPath, undefined, cleanupSource === true);
+        await jobQueue.addOrganizeJob(
+          requestId,
+          audiobookId,
+          normalizedPath,
+          undefined,
+          cleanupSource === true,
+          validatedFiles.length > 0 ? validatedFiles : undefined
+        );
 
-        logger.info(`Manual import queued: request=${requestId}, path=${normalizedPath}, audioFiles=${audioCheck.count}`);
+        logger.info(`Manual import queued: request=${requestId}, path=${normalizedPath}, audioFiles=${audioFileCount}`);
 
         return NextResponse.json({
           success: true,